A) Field of the Invention
The present invention relates to a laser device and more particularly to a wavelength tunable laser device.
B) Description of the Related Art
In order to cope with increasing data traffic, a wavelength division multiplexing (WDM) optical communication system has been developed and is in practical use, which transmits optical signals of a plurality of wavelengths via a single optical fiber at a time. In a conventional WDM optical communication system, sophisticated processes such as optical add drop multiplexer (OADM), wavelength routing and optical packet transmission have been studied in order to realize a large capacity flexible system by positively utilizing the wavelength information of an optical signal. In order to realize such processes, a wavelength tunable laser of a single device having a wide wavelength tunable range and a high output power has been desired as a light source.
Japanese Patent Laid-open Publication No. 2003-283024 proposes a novel structure of a wavelength tunable laser of a single device and a high output having a wide wavelength tunable range.
FIG. 5A shows the structure proposed in Japanese Patent Laid-open Publication No. 2003-283024. A gain medium 54 having an optical gain in a wide wavelength range, a band-pass filter 55 and an etalon filter 56 are disposed along an optical axis 57 between a reflection mirror 51 and a semi-transparent mirror 52. The reflection mirror 51 and semi-transparent mirror 52 define a cavity 50. The gain medium 54 is structured, for example, by a semiconductor optical amplifier (SOA). The band-pass filter 55 is structured, for example, by an acousto-optical tunable filter (AOTF). A frequency controller 58 controls the pass band of the band-pass filter 55. The etalon filter 56 is structured by a Fabry-Perot etalon filter. The etalon filter is designed and disposed so that it has a periodical sharp transmission (wavelength) spectrum, for example, satisfying the specifications of an International Telecommunications Union (ITU) grid.
Optical gain extending over a wide frequency range are generated by the gain medium 54, and only narrow band energies are filtered by the band-pass filter 55, and further filtered by the etalon filter having transmission wavelengths of the periodical sharp transmission characteristics. Resonance is formed by the cavity. An SOA-AOTF laser using such SOA and AOTF generates a high output laser beam in a single mode.
The specific structure and measured performances of such an SOA-AOTF laser was pronounced by K. Takabayashi et al. in Proc. of ECOC 2003, vol. 4, 890.
FIG. 5B shows the structure of an SOA-AOTF laser. The layout of this structure has the reversed right and left of the structure shown in FIG. 5A. A semiconductor optical amplifier SOA is used as the gain medium 54, whose one facet constitutes the semi-transparent mirror 52. The band-pass filter 55 and Fabry-Perot etalon 56 are disposed between SOA and the reflection mirror 51.
The band-pass filter 55 is constituted of two AOTF having a symmetrical structure to eliminate a Doppler shift. A lens 59 is disposed between the band-pass filter 55 and Fabry-Perot etalon 56 to improve an optical coupling efficiency. The cavity 50 defined between the semi-transparent mirror 52 and reflection mirror 51 has a length of 50 mm. About 90 nm of a wavelength tunable width Δλ is obtained.
A combination of these three devices can be considered promising: a device having an optical gain over a wide wavelength range; a wavelength filter having a sharp (periodical, fixed or finely adjusted) wavelength selectivity; and a wavelength filter having a wide wavelength tunable range with a coarse wavelength selectivity.
Japanese Patent Laid-open Publication No. HEI-6-29628 proposes an optical coupler and a semiconductor laser made of a gain section and a wavelength selection coupler integrated together. Lower and upper waveguides are formed in a semiconductor structure, and a diffraction grating is formed above the lower and upper waveguides to provide selective coupling between the lower and upper waveguides. The expected filter band width is described as 2.5 nm.
IEEE Photonics Technology Letters, vol. 7, no. 7, (1995) 697–699 propose a grating assisted codirectional coupler laser with super structure grating reflector (GCSR) laser having the above-described three functions integrated together. A gain section, a coupler section, a phase control section and a reflection section are formed on an InP substrate and electrodes are formed independently for each section. The coupler section has upper and lower waveguides and a diffraction grating formed above the upper and lower waveguides, similar to the wavelength selection coupler of Japanese Patent Laid-open Publication No. HEI-6-29628, and realizes the wavelength tunable filter function by utilizing the phenomenon that light in a specific wavelength range determined by an equivalent refractive index difference between two waveguides controllable by current injection is selectively moved between two waveguides. The reflection section is structured by a superstructure grating reflector and has the periodical sharp wavelength filer characteristics. The transmission peak wavelength of this filter can also be finely adjusted by current injection. A single longitudinal mode oscillation can be realized and a wide wavelength tunable range Δλ of 100 nm can be obtained.